Consider using LEDs to light a one-acre farm on Mars. Good solar panels have an efficiency of 22 percent, and great LEDs have an efficiency of 35 percent. With Mars’ solar irradiance of 59 percent, you would need a minimum of 22 acres of solar panels for the plants to receive the same amount of light as on Earth.
For a small outpost, that is doable. But to feed Promise’s population of 304,667, you would need 2,500 acres of greenhouse, which in turn would require you to build 55,000 acres of solar panels. That’s 86 square miles, or a twentieth of all solar panels ever made. And that’s for the lights alone. Luckily, no one farmed that way on Mars.
Having finished my count, I moved my hand down from where it blocked the sun’s glare. I wrote “174 wheat plants per meter” in the top left corner of my notebook. Hmm. Well within normal. No decaying plant matter; no smell of rot. None of the cracks in the concrete floor had more than a two-millimeter gap. Where was all the oxygen going?
I was an accountant with the Mars Accountants and Resource Society. I was also confused. Greenhouse 211 produced less oxygen than the math said it should. In fact, it produced the least of all 309 greenhouses.
Oxygen accounting for a greenhouse is harder than for other facilities. Plants are unpredictable, and their oxygen production varies day to day. But in the end, a certain weight of product should equate to a certain weight of oxygen.
I walked over to a farmer filling a crack in the cement floor with rubber sealant. Greenhouses are softsuit areas, so it took a gentle prod to get his attention.
I asked, “Any trouble with the cracks lately?”
He straightened up and turned to squarely face me.
“I’m not hiding anything from MARS.”
Somewhat disgruntled that he could tell I worked for the Mars Accountants and Resource Society, I said, “I’m not making any accusations. I’m just looking to do my accounting.”
“That’s probably what you told the foreman over at Greenhouse 19.”
He was referring to a case two years ago involving a foreman with a three-millimeter crack in the floor of his greenhouse. The floors were made from alternating layers of rubber sealant and cement. As long as the cracks were small enough, the rubber would stretch and still hold a seal.
The foreman at Greenhouse 19 hadn’t wanted to dig a trench all 350 feet across his concrete floor to fix the crack. Instead, his workers filled it in with rubber the same way small cracks were repaired. They then hid it with cement. The crack caused a 2 percent leak in the oxygen seal. It was so well hidden that it took my coworker, Ryan Decker, a full four weeks to track it down.
I said, “We wouldn’t have cared about the leak if he hadn’t fudged his production numbers to cover it up. His boss may have cared, but at MARS, we only record where the oxygen goes. We don’t make judgments about it.”
Stooping to unclip a sheet of reflective Mylar from its post and gently fold it off to the side, the farmer ran his thumb along the crack.
“How many millimeters?” I asked.
“1.1” He looked up, expecting me to argue with his measuring method. I had measured enough concrete cracks in my five years as an accountant on Mars not to doubt him. My current accuracy was within 0.2 millimeters, but he did this far more than I did.
“Thank you,” I said, writing the measurement down in my notepad. That seemed to satisfy him.
Coming to the four-foot-wide row of grain, the farmer lifted a section of the grow tray that was filled with soil and wheat and set it off to the side. On Earth, it would have been over 130 pounds; here on Mars, it weighed only fifty pounds.
Continuing with the rubber sealant, he filled the crack until he came to the next row of Mylar reflectors. The whole greenhouse was laid out like this. It alternated four feet of reflectors, four feet of wheat, another four feet of reflectors, and then a two-foot walking path.
I decided to ask him a question; people always seemed happy to explain to an outsider why they were stupid.
“I always wonder why the greenhouses are laid out in such long rows. It’s 3,200 feet by 350 feet. That’s rather long and skinny. Wouldn’t it be better just to build them more square?”
“Geometry.”
Not really an answer. I waited until he came to the conclusion he needed to tell me more.
“The problem with solar reflectors,” he said as he waved his hand at the 3,200-foot-long row of four-foot-wide shiny foil, “is that the sun moves throughout the day, and it would take heavy, complicated machinery to keep a reflector pointed at the same spot in the field all day. As you can see here, there is nothing complicated about our reflectors. Just Mylar rectangles held at the correct angle by posts in the corners. A combination of the greenhouses running east to west and the reflectors being directly next to the wheat means most of the light hitting the reflectors hits the wheat. Works out to each plant getting a little over double the sunlight it otherwise would on Mars. Brings the light level up to about 1.1 of Earth-normal.”
“What about the plants at the ends?” I asked.
“They don’t receive as much light. That’s why the greenhouses are so long. It’s so there are fewer ends. We plant leafy greens there since they are more tolerant of the low-light levels.”
I said, “So, you never have to move the reflectors?” This seemed unlikely, as the only thing farmers ever seemed to do was move reflectors.
“On a daily basis, no, but over the course of a year, the sun changes its angle in the sky by over fifty degrees. Every four weeks, we have to come through. . .
